Noise manifests itself as sound. We could say that noise contains all the undesired sounds we need to get rid of in some way or another (even though in some cases a noise may be added to the audio signal purposefully to obtain certain desired results). There are many types of noise and many causes that generate it, so each and every time different solutions are adopted to eliminate it (or at least attenuate it). In the first part of the chapter we'll be taking a look at the principle causes of noise and the influence they have on the audio signal.